Activated sludge processes have been used to remove biological oxygen demand (BOD) from municipal sewage, industrial wastewaters and the like. In such a process, a wastewater influent is mixed with a microorganism-containing recycled biomass or activated sludge in an initial contact zone to form a mixed liquor. At some point in the process, the mixed liquor is aerated with sufficient oxygen to grow and maintain a satisfactory population of microorganisms which sorb, assimilate and metabolize the BOD of the wastewater.
In the activated sludge process disclosed in U.S. Pat. No. 3,964,998, wastewater and recycled activated sludge are mixed with mechanical stirrers in a first stage which is operated under anoxic conditions. The mixed liquor is subsequently aerated in a second stage, subjected to anoxic conditions in a third stage, aerated in a fourth stage and then clarified to separate an activated sludge.
In another process, wastewater and recycled activated sludge are mixed and circulated around a plurality of concentric, annular basins or channels by a plurality of surface aeration discs or other mechanical surface aeration devices which churn oxygen into the upper surface of the mixed liquor and provide sufficient agitation to prevent settling. The mixed liquor flows from one channel to the next and finally is introduced into a clarifier to separate an activated sludge. The channels can be operated as a series of complete mix reactors so that the dissolved oxygen content in the first channel in which the wastewater and recycled activated sludge is initially mixed is about zero or less and the dissolved oxygen content is subsequently increased as the mixed liquor moves from one channel to the next.